This invention relates to radio frequency amplifiers with gain control, and particularly to such amplifiers using field effect transistors. There are many methods of gain control in radio frequency amplifiers that are known in the prior art. The amplifier of this invention uses a simple and unique circuit to combine two of such methods in an advantageous manner.
It is well known in the prior art that the gain of a field effect transistor which is biased near pinch-off may be reduced by reducing the source to gate voltage and thus driving the operating point of the FET into an area of decreasing gain. This has generally been accomplished in the prior art by varying the gate voltage, even though this introduces the necessity of an opposite polarity power supply to vary this voltage below ground and further may require adjustments or additional circuitry to take care of non-uniformity of pinch-off voltage in mass production. Prior art circuits in which the source voltage was varied in order to achieve gain control generally required a current source with a small impedance in the gain control circuit in order to supply sufficient current across the biasing resistor to raise the source voltage of the FET. However, the small impedance of the current source would serve as a shunt to RF signal even when gain reduction was not desired. If a higher impedance were used, however, undesirably high supply voltage was required. Therefore, the gate bias was usually varied in spite of its already mentioned difficulties.
In addition, the method of reducing gain by varying the source to gate voltage is limited in range by distortion due to overload as the gain is reduced, no matter which of the source or gate voltages is varied. This is due to the fact that, as input signal goes up in strength, AGC causes more gain reduction in the FET, so that at some point the FET becomes over-loaded by the strong input signal.
Another approach to gain control of a field effect transistor amplifier is to shunt a controllable portion of the input RF signal away from the amplifier. It is long been known that some semiconductor diodes exhibit dynamic impedance for radio frequency signals which varies with direct current therethrough. PIN diodes have been created to advantageously use this effect; and such diodes have been used in a shunt path for an RF amplifier as shown in the U.S. Pat. No. 4,019,160 to Kam. This method of gain control by itself, however, does not make full use of the gain control possibilities achievable with field effect transistors.
If the shunt path technique of gain control is used in combination with the pinch-off method described above, the former assists in reducing the overload problems of the latter by decreasing the input signal to the FET at the same time that the gain of the FET itself is reduced. This extends the AGC range of the FET as its overload characteristics are improved. If, in addition, the source to gate voltage could be controlled by varying the source voltage rather than the gate voltage, a simpler power supply could be used.